1. Field of the Invention
The present invention relates to an exhaust gas purification apparatus for purifying exhaust gas by performing an exhaust gas purification reaction on a catalyst by use of a designated additive, as typified by the urea-SCR (selective reduction) system, and more specifically relates to an exhaust gas purification apparatus that promotes, through a catalyst, an exhaust gas purification reaction by use of an additive such as a urea solution injected from an additive injection valve, to thereby purify exhaust gas.
2. Description of Related Art
Various urea-SCR systems are under development with expectation of purifying NOx contained in exhaust gas at high purification rate for power plants, factories, vehicles (especially, diesel engine vehicles), etc., and some of them are in practical use. A structure of a conventional urea-SCR apparatus (system) as disclosed in Japanese Patent Application Laid-open No. 2003-293739 is explained below with reference to FIG. 17.
As shown in FIG. 17, this apparatus is constituted mainly by a catalyst 51 for promoting an exhaust gas purification reaction, an exhaust pipe 52 for guiding exhaust gas discharged from an exhaust gas emission source (internal combustion engine, for example) to the catalyst 51, and an additive injection valve 53 disposed midway of the exhaust pipe 52 for injecting a urea solution as an additive into the exhaust gas flowing through the exhaust pipe 52. The catalyst 51 is for promoting a reduction reaction (exhaust gas purification reaction) of NOx. The additive injection valve 53 is disposed with inclination with respect to the exhaust pipe 52 so that an injection orifice 53a thereof is directed to the side of the catalyst 51.
The apparatus having such a configuration operates to inject the urea solution into the exhaust gas flowing through the exhaust pipe 52 from the additive injection valve 53, and supply the catalyst 51 disposed downstream of the additive injection valve 53 with the urea solution together with the exhaust gas utilizing the flow of the exhaust gas in order to purify the exhaust gas through the reduction reaction of NOx on the catalyst 51. More particularly, the urea solution is hydrolyzed by the heat of the exhaust gas, as a result of which ammonia (NH3) is produced, and this ammonia is added to NOx contained in the exhaust gas, which has been selectively adsorbed by the catalyst. And then, the NOx is reduced by the ammonia to be purified on the catalyst 51.
Generally, in such a urea-SCR apparatus, the efficiency of the exhaust gas purification reaction increases as the mixture rate of the ammonia produced by the hydrolysis of the urea solution to the exhaust gas to be purified increases. If the hydrolysis of the urea solution performed upstream of the catalyst 51 is insufficient, and accordingly the amount of the ammonia is not sufficient to purify (reduce) the NOx contained in the exhaust gas, the purification capacity of the urea-SCR apparatus is lowered, which causes exhaust emissions to increase. For example, this can occur when a vehicle provided with the urea-SCR apparatus is running at high speed while discharging a large amount of exhaust gas from its engine, causing the exhaust gas to flow at high speed. In this state, a time period between a moment at which the urea solution is injected and a moment at which the injected urea solution is added to the catalyst 51 is short. This make it difficult for the ammonia gas and the exhaust gas to sufficiently mix with each other so that ammonia is produced by an amount necessary to purify the NOx.
To cope with such a problem, there is proposed a urea-SCR apparatus having a long exhaust pipe for the purpose of increasing the distance between a position at which the urea solution (additive) is injected and a position at which the injected urea solution carried by the exhaust gas is added to the catalyst, to thereby provide a sufficiently long time period for the mixture and hydrolysis. There is also proposed a urea-SCR apparatus having a muffler carrying therein a hydrolysis catalyst, which is disposed upstream of the catalyst for the purpose of promoting the hydrolysis of the urea solution, and consequently promoting the production of ammonia.
However, the former apparatus has a problem in that it is unavoidable to upsize the system because of its long exhaust pipe. On the other hand, the latter apparatus has a problem in that the system increases in complexity, and significant design change on the overall system may be required.